This invention is directed to plant genetic engineering. In particular, it relates to modulating seed (and in particular endosperm, embryo and seed coat) development in plants.
A fundamental problem in biology is to understand how seed development. In flowering plants, the ovule generates the female gametophyte which is composed of egg, central, synergid and antipodal cells (Reiser, et al., Plant Cell, 1291-1301 (1993)). All are haploid except the central cell which contains two daughter nuclei that fuse prior to fertilization. One sperm nucleus fertilizes the egg to form the zygote, whereas another sperm nucleus fuses with the diploid central cell nucleus to form the triploid endosperm nucleus (van Went, et al., Embryology of Angiosperms, pp. 273-318 (1984)). The two fertilization products undergo distinct patterns of development. In Arabidopsis, the embryo passes through a series of stages that have been defined morphologically as preglobular, globular, heart, cotyledon and maturation (Goldberg, R. B., et al., Science (1994) 266: 605-614; Mansfield, S. G., et al., Arabidopsis: An Atlas of Morphology and Development, pp. 367-383 (1994)). The primary endosperm nucleus undergoes a series of mitotic divisions to produce nuclei that migrate into the expanding central cell (Mansfield, S. G., et al., Arab Inf Serv 27: 53-64 (1990); Webb, M. C., et al., Planta 184:187-195 (1991)). Cytokinesis sequesters endosperm cytoplasm and nuclei into discrete cells (Mansfield, S. G., et al., Arab Inf Serv 27:65-72 (1990)) that produce storage proteins, starch, and lipids which support embryo growth (Lopes, M. A. et al., Plant Cell 5:1383-1399 (1993)). Fertilization also activates development of the integument cell layers of the ovule that become the seed coat, and induces the ovary to grow and form the fruit, or silique, in Arabidopsis.
Of particular interest are recent discoveries of genes that control seed, and in particular endosperm, development. For instance, MEDEA (MEA) (also known as FIE1 (see, e.g., copending U.S. patent application Ser. No. 09/071,838) and F644 (see, e.g., Kiyosue T, et al. (1 999) Proc Natl Acad Sci USA 96(7):4186-91) encodes an Arabidopsis SET domain polycomb protein that appears to play a role in endosperm development. Inheritance of a maternal loss-of-function mea allele results in embryo abortion and prolonged endosperm production, irrespective of the genotype of the paternal allele. Thus, only the maternal wild-type MEA allele is required for proper embryo, endosperm, and seed coat development (Kinoshita T, et al. (1999) Plant Cell 10:1945-52). These results reveal functions for plant polycomb proteins in the suppression of central cell proliferation and endosperm development (Kiyosue T, et al. supra).
Another gene product that controls seed development is FIE, also known as FIE3 (see, e.g., copending U.S. patent application Ser. No. 09/071,838). The FIE protein is a homolog of the WD motif-containing Polycomb proteins from Drosophila and mammals (Ohad, N. et al. Plant Cell 11(3):407-16 (1999)). In Drosophila, these proteins function as repressors of homeotic genes. A female gametophyte with a loss-of-function allele of fie undergoes replication of the central cell nucleus and initiates endosperm development without fertilization. These results suggest that the FIE Polycomb protein functions to suppress a critical aspect of early plant reproduction, namely, endosperm development, until fertilization occurs.
Control of the expression of genes that control egg and central cell differentiation, or those that control reproductive development, i.e. embryo, endosperm and seed coat, is useful in the production of plants with a range of desired traits. These and other advantages are provided by the present application.
This invention provides isolated nucleic acids comprising a polynucleotide sequence, or its complement, encoding an ATR polypeptide exhibiting at least 60% sequence identity to SEQ ID NO:2. For instance, the nucleic acid can encode the ATR polypeptide displayed in SEQ ID NO:2. In one aspect, the polynucleotide sequence comprises SEQ ID NO:5 or SEQ ID NO:1. In some aspects of the invention, the nucleic acid further comprises a promoter operably linked to the polynucleotide. In some embodiments, the promoter is constitutive. In other embodiments, the promoter is from an ATR gene. For example, the promoter can comprise a polynucleotide at least 70% identical to SEQ ID NO:3. In some aspects, the promoter comprises SEQ ID NO:3. In some aspects of this invention, the promoter further comprises a polynucleotide at least 70% identical to SEQ ID NO:4. For example, in some aspects the promoter comprises SEQ ID NO:4. In some aspects, the polynucleotide sequence is linked to the promoter in an antisense orientation.
The invention also provides an isolated nucleic acid molecule comprising a polynucleotide sequence exhibiting at least 60% sequence identity to SEQ ID NO:1.
The invention also provides an expression cassette comprising a promoter operably linked to a heterologous polynucleotide sequence, or complement thereof, encoding an ATR polypeptide exhibiting at least 60% sequence identity to SEQ ID NO:2. For instance, the nucleic acid can encode the ATR polypeptide displayed in SEQ ID NO:2. In some aspects, the polynucleotide sequence comprises SEQ ID NO:5 or SEQ ID NO:1. In some aspects of the invention, the nucleic acid further comprises a promoter operably linked to the polynucleotide. In some embodiments, the promoter is constitutive. In other embodiments, the promoter is from an ATR gene. For example, the promoter can comprise a polynucleotide at least 70% identical to SEQ ID NO:3. In some aspects, the promoter comprises SEQ ID NO:3. In some aspects of this invention, the promoter further comprises a polynucleotide at least 70% identical to SEQ ID NO:4. For example, in some aspects the promoter comprises SEQ ID NO:4. In some aspects, the polynucleotide sequence is linked to the promoter in an antisense orientation.
The invention also provides an expression cassette for the expression of a heterologous polynucleotide in a plant cell. In some aspects, the expression cassette comprises a promoter polynucleotide at least 70% identical to SEQ ID NO:3 that is operably linked to a heterologous polynucleotide. In some aspects, the promoter comprises SEQ ID NO:3. In some aspects, the promoter further comprises a polynucleotide at least 70% identical to SEQ ID NO:4. For instance, in some embodiments, the promoter comprises SEQ ID NO:4.
The present invention also provides a host cell comprising an exogenous polynucleotide sequence comprising a polynucleotide sequence, or complement thereof, encoding an ATR polypeptide exhibiting at least 60% sequence identity to SEQ ID NO:2. In some aspects of the invention, the nucleic acid further comprises a promoter operably linked to the polynucleotide sequence. In some aspects, the promoter is constitutive. In some aspects, the promoter comprises a polynucleotide at least 70% identical to SEQ ID NO:3. The promoter, for instance, can comprise SEQ ID NO:3. In some aspects, the promoter further comprises a polynucleotide at least 70% identical to SEQ ID NO:4. For instance, in some embodiments, the promoter comprises SEQ ID NO:4. In some aspects, the promoter is operably linked to the exogenous polynucleotide sequence in an antisense orientation.
The present invention also provides an isolated polypeptide comprising an amino acid sequence at least 60% identical to SEQ ID NO:2 and capable of exhibiting at least one biological activity of the polypeptide displayed in SEQ ID NO:2, or fragment thereof. The present invention also provides for an antibody capable of binding such polypeptides.
The present invention also provides a method of introducing an isolated nucleic acid into a host cell comprising, (a) providing an isolated nucleic acid or its complement, encoding an ATR polypeptide exhibiting at least 60% sequence identity to SEQ ID NO:2. and (b) contacting the nucleic acid with the host cell under conditions that permit insertion of the nucleic acid into the host cell.
The present invention also provides a method of modulating transcription, comprising introducing into a host cell an expression cassette comprising a promoter operably linked to a heterologous ATR polynucleotide, the heterologous ATR polynucleotide encoding an ATR polypeptide at least 60% identical to SEQ ID NO:2, and detecting a host cell with modulated transcription. In some aspects of the invention, the heterologous ATR polynucleotide encodes SEQ ID NO:2. In some aspect, the polynucleotide sequence comprises SEQ ID NO:5 or SEQ ID NO:1. In some aspects, the expression cassette is introduced into a host cell by Agrobacterium. In some aspects, the expression cassette is introduced by a sexual cross. In some aspects of the method of the invention, modulating transcription results in the modulation of endosperm development in a plant. In some aspects, endosperm development is enhanced. In other aspects, endosperm development is decreased. In some aspects of the methods of the invention, the promoter is operably linked to the ATR polynucleotide in an antisense orientation.
The present invention also provides a method of detecting a nucleic acid in a sample, comprising (a) providing an isolated nucleic acid molecule comprising a polynucleotide sequence, or its complement, encodinga R polypeptide exhibiting at least 60% sequence identity to SEQ ID NO:2., (b) contacting the isolated nucleic acid molecule with a sample under conditions that permit a comparison of the sequence of the isolated nucleic acid molecule with the sequence of DNA in the sample, and (c) analyzing the result of the comparison. In some aspects of the method, the isolated nucleic acid molecule and the sample are contacted under conditions that permit the formation of a duplex between complementary nucleic acid sequences.
The present invention also provides a transgenic plant cell or transgenic plant comprising a polynucleotide sequence, or its complement, encoding an ATR polypeptide exhibiting at least 60% sequence identity to SEQ ID NO:2. For instance, the nucleic acid can encode the ATR polypeptide displayed in SEQ ID NO:2. In one aspect, the polynucleotide sequence comprises SEQ ID NO:5 or SEQ ID NO:1. In some aspects of the invention, the nucleic acid further comprises a promoter operably linked to the polynucleotide. In some embodiments, the promoter is constitutive. In other embodiments, the promoter comprises a polynucleotide at least 70% identical to SEQ ID NO:3. In some aspects, the promoter comprises SEQ ID NO:3. In some aspects of this invention, the promoter further comprises a polynucleotide at least 70% identical to SEQ ID NO:4. For example, in some aspects the promoter comprises SEQ ID NO:4. In some aspects, the polynucleotide sequence is linked to the promoter in an antisense orientation. The present invention also provides a plant that is regenerated from a plant cell as described above.
The phrase xe2x80x9cnucleic acid sequencexe2x80x9d refers to a single or double-stranded polymer of deoxyribonucleotide or ribonucleotide bases read from the 5xe2x80x2 to the 3xe2x80x2 end. It includes chromosomal DNA, self-replicating plasmids, infectious polymers of DNA or RNA and DNA or RNA that performs a primarily structural role.
A xe2x80x9cpromoterxe2x80x9d is defined as an array of nucleic acid control sequences that direct transcription of an operably linked nucleic acid. As used herein, a xe2x80x9cplant promoterxe2x80x9d is a promoter that functions in plants. Promoters include necessary nucleic acid sequences near the start site of transcription, such as, in the case of a polymerase II type promoter, a TATA element. A promoter also optionally includes distal enhancer or repressor elements, which can be located as much as several thousand base pairs from the start site of transcription. A xe2x80x9cconstitutivexe2x80x9d promoter is a promoter that is active under most environmental and developmental conditions. An xe2x80x9cinduciblexe2x80x9d promoter is a promoter that is active under environmental or developmental regulation. The term xe2x80x9coperably linkedxe2x80x9d refers to a functional linkage between a nucleic acid expression control sequence (such as a promoter, or array of transcription factor binding sites) and a second nucleic acid sequence, wherein the expression control sequence directs transcription of the nucleic acid corresponding to the second sequence.
The term xe2x80x9cplantxe2x80x9d includes whole plants, plant organs (e.g., leaves, stems, flowers, roots, etc.), seeds and plant cells and progeny of same. The class of plants which can be used in the method of the invention is generally as broad as the class of flowering plants amenable to transformation techniques, including angiosperms (monocotyledonous and dicotyledonous plants), as well as gymnosperms. It includes plants of a variety of ploidy levels, including polyploid, diploid, haploid and hemizygous.
A polynucleotide sequence is xe2x80x9cheterologous toxe2x80x9d an organism or a second polynucleotide sequence if it originates from a foreign species, or, if from the same species, is modified from its original form. For example, a promoter operably linked to a heterologous coding sequence refers to a coding sequence from a species different from that from which the promoter was derived, or, if from the same species, a coding sequence which is different from any naturally occurring allelic variants.
A polynucleotide xe2x80x9cexogenous toxe2x80x9d an individual plant is a polynucleotide which is introduced into the plant, or a predecessor generation of the plant, by any means other than by a sexual cross. Examples of means by which this can be accomplished are described below, and include Agrobacterium-mediated transformation, biolistic methods, electroporation, in planta techniques, and the like.
The phrase xe2x80x9chost cellxe2x80x9d refers to a cell from any organism. Preferred host cells are derived from plants, bacteria, yeast, fungi, insects or other animals. Methods for introducing polynucleotide sequences into various types of host cells are well known in the art.
The xe2x80x9cbiological activity of a polypeptidexe2x80x9d refers to any molecular activity or phenotype that is caused by the polypeptide. For example, the ability to transfer a phosphate to a substrate or the ability to bind a specific DNA sequence is a biological activity. One biological activity of ATR is the ability to modulate endosperm production in plants.
An xe2x80x9cexpression cassettexe2x80x9d refers to a nucleic acid construct, which when introduced into a host cell, results in transcription and/or translation of an RNA or polypeptide, respectively. Antisense or sense constructs that are not or cannot be translated are expressly included by this definition.
A xe2x80x9cATR nucleic acidxe2x80x9d or xe2x80x9cATR polynucleotide sequencexe2x80x9d of the invention is a subsequence or full length polynucleotide sequence of a gene which encodes a polypeptide involved in control of reproductive development and which, when the maternal allele is mutated, allows for increased production of the endosperm and/or abortion of the embryo. In some embodiments, the polypeptides of the invention have a nuclear localization signal. An exemplary nucleic acid of the invention is the Arabidopsis ATR sequence disclosed below. ATR polynucleotides are defined by their ability to hybridize under defined conditions to the exemplified nucleic acids or PCR products derived from them. An ATR polynucleotide is typically at least about 30-40 nucleotides to about 7000, usually less than about 10000 nucleotides in length. The nucleic acids contain coding sequence of from about 100 to about 4000 nucleotides, often from about 500 to about 3600 nucleotides in length.
ATR nucleic acids are a new class of plant regulatory genes that encode polypeptides with sequence identity to members of the endonuclease III genes found in a diverse collection of organisms. Endonuclease III is implicated in various DNA repair reactions. Thus proteins related to endonuclease III are likely to have a chromosomal function. ATR (SEQ ID NO:1) is most related to endonuclease III from Deinococcus radiodurans Genbank Accession No. AE002073 (see, e.g., White, O. et al. Science 286:1571-1577 (1999)). ATR polynucleotides can also encode abipartite nuclear localization signal (e.g., amino acid positions 43-60 and 61-78 in SEQ ID NO:2) comprised of basic amino acids. ATR polypeptides also contain a leucine zipper sequence (e.g., positions 1330-1351 of SEQ ID NO:2).
In the case of both expression of transgenes and inhibition of endogenous genes (e.g., by antisense, or sense suppression) one of skill will recognize that the inserted polynucleotide sequence need not be identical, but may be only xe2x80x9csubstantially identicalxe2x80x9d to a sequence of the gene from which it was derived. As explained below, these substantially identical variants are specifically covered by the term ATR nucleic acid.
In the case where the inserted polynucleotide sequence is transcribed and translated to produce a functional polypeptide, one of skill will recognize that because of codon degeneracy a number of polynucleotide sequences will encode the same polypeptide. These variants are specifically covered by the terms xe2x80x9cATR nucleic acidxe2x80x9d. In addition, the term specifically includes those sequences substantially identical (determined as described below) with an ATR polynucleotide sequence disclosed here and that encode polypeptides that are either mutants of wild type ATR polypeptides or retain the function of the ATR polypeptide (e.g., resulting from conservative substitutions of amino acids in the ATR polypeptide). In addition, variants can be those that encode dominant negative mutants as described below.
Two nucleic acid sequences or polypeptides are said to be xe2x80x9cidenticalxe2x80x9d if the sequence of nucleotides or amino acid residues, respectively, in the two sequences is the same when aligned for maximum correspondence as described below. The terms xe2x80x9cidenticalxe2x80x9d or percent xe2x80x9cidentity,xe2x80x9d in the context of two or more nucleic acids or polypeptide sequences, refer to two or more sequences or subsequences that are the same or have a specified percentage of amino acid residues or nucleotides that are the same, when compared and aligned for maximum correspondence over a comparison window, as measured using one of the following sequence comparison algorithms or by manual alignment and visual inspection. When percentage of sequence identity is used in reference to proteins or peptides, it is recognized that residue positions that are not identical often differ by conservative amino acid substitutions, where amino acids residues are substituted for other amino acid residues with similar chemical properties (e.g., charge or hydrophobicity) and therefore do not change the functional properties of the molecule. Where sequences differ in conservative substitutions, the percent sequence identity may be adjusted upwards to correct for the conservative nature of the substitution. Means for making this adjustment are well known to those of skill in the art. Typically this involves scoring a conservative substitution as a partial rather than a full mismatch, thereby increasing the percentage sequence identity. Thus, for example, where an identical amino acid is given a score of 1 and a non-conservative substitution is given a score of zero, a conservative substitution is given a score between zero and 1. The scoring of conservative substitutions is calculated according to, e.g., the algorithm of Meyers and Miller, Computer Applic. Biol. Sci. 4:11-17 (1988) e.g., as implemented in the program PC/GENE (Intelligenetics, Mountain View, Calif., USA).
The phrase xe2x80x9csubstantially identical,xe2x80x9d in the context of two nucleic acids or polypeptides, refers to a sequence or subsequence that has at least 40% sequence identity with a reference sequence. Alternatively, percent identity can be any integer from 40% to 100%. More preferred embodiments include at least: 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 99%. compared to a reference sequence using the programs described herein; preferably BLAST using standard parameters, as described below. This definition also refers to the complement of a test sequence, when the test sequence has substantial identity to a reference sequence.
For sequence comparison, typically one sequence acts as a reference sequence, to which test sequences are compared. When using a sequence comparison algorithm, test and reference sequences are entered into a computer, subsequence coordinates are designated, if necessary, and sequence algorithm program parameters are designated. Default program parameters can be used, or alternative parameters can be designated. The sequence comparison algorithm then calculates the percent sequence identities for the test sequences relative to the reference sequence, based on the program parameters.
A xe2x80x9ccomparison windowxe2x80x9d, as used herein, includes reference to a segment of any one of the number of contiguous positions selected from the group consisting of from 20 to 600, usually about 50 to about 200, more usually about 100 to about 150 in which a sequence may be compared to a reference sequence of the same number of contiguous positions after the two sequences are optimally aligned. Methods of alignment of sequences for comparison are well-known in the art. Optimal alignment of sequences for comparison can be conducted, e.g., by the local homology algorithm of Smith and Waterman, Adv. Appl. Math. 2:482 (1981), by the homology alignment algorithm of Needleman and Wunsch, J. Mol. Biol. 48:443 (1970), by the search for similarity method of Pearson and Lipman, Proc. Nat""l. Acad. Sci. USA 85:2444 (1988), by computerized implementations of these algorithms (GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics Software Package, Genetics Computer Group, 575 Science Dr., Madison, Wis.), or by manual alignment and visual inspection.
One example of a useful algorithm is PILEUP. PILEUP creates a multiple sequence alignment from a group of related sequences using progressive, pairwise alignments to show relationship and percent sequence identity. It also plots a tree or dendogram showing the clustering relationships used to create the alignment. PILEUP uses a simplification of the progressive alignment method of Feng and Doolittle, J. Mol. Evol. 35:351-360 (1987). The method used is similar to the method described by Higgins and Sharp, CABIOS 5:151-153 (1989). The program can align up to 300 sequences, each of a maximum length of 5,000 nucleotides or amino acids. The multiple alignment procedure begins with the pairwise alignment of the two most similar sequences, producing a cluster of two aligned sequences. This cluster is then aligned to the next most related sequence or cluster of aligned sequences. Two clusters of sequences are aligned by a simple extension of the pairwise alignment of two individual sequences. The final alignment is achieved by a series of progressive, pairwise alignments. The program is run by designating specific sequences and their amino acid or nucleotide coordinates for regions of sequence comparison and by designating the program parameters. For example, a reference sequence can be compared to other test sequences to determine the percent sequence identity relationship using the following parameters: default gap weight (3.00), default gap length weight (0.10), and weighted end gaps.
Another example of algorithm that is suitable for determining percent sequence identity and sequence similarity is the BLAST algorithm, which is described in Altschul et al., J. MoL Biol. 215:403-410 (1990). Software for performing BLAST analyses is publicly available through the National Center for Biotechnology Information (http://www.ncbi.nlm.nih.gov/). This algorithm involves first identifying high scoring sequence pairs (HSPs) by identifying short words of length W in the query sequence, which either match or satisfy some positive-valued threshold score T when aligned with a word of the same length in a database sequence. T is referred to as the neighborhood word score threshold (Altschul et al, supra). These initial neighborhood word hits act as seeds for initiating searches to find longer HSPs containing them. The word hits are extended in both directions along each sequence for as far as the cumulative alignment score can be increased. Extension of the word hits in each direction are halted when: the cumulative alignment score falls off by the quantity X from its maximum achieved value; the cumulative score goes to zero or below, due to the accumulation of one or more negative-scoring residue alignments; or the end of either sequence is reached. The BLAST algorithm parameters W, T, and X determine the sensitivity and speed of the alignment. The BLAST program uses as defaults a wordlength (W) of 11, the BLOSUM62 scoring matrix (see Henikoff and Henikoff, Proc. Natl. Acad. Sci. USA 89:10915 (1989)) alignments (B) of 50, expectation (E) of 10, M=5, N=xe2x88x924, and a comparison of both strands.
The BLAST algorithm also performs a statistical analysis of the similarity between two sequences (see, e.g., Karlin and Altschul, Proc. Nat""l. Acad. Sci. USA 90:5873-5787 (1993)). One measure of similarity provided by the BLAST algorithm is the smallest sum probability (P(N)), which provides an indication of the probability by which a match between two nucleotide or amino acid sequences would occur by chance. For example, a nucleic acid is considered similar to a reference sequence if the smallest sum probability in a comparison of the test nucleic acid to the reference nucleic acid is less than about 0.2, more preferably less than about 0.01, and most preferably less than about 0.001.
xe2x80x9cConservatively modified variantsxe2x80x9d applies to both amino acid and nucleic acid sequences. With respect to particular nucleic acid sequences, conservatively modified variants refers to those nucleic acids which encode identical or essentially identical amino acid sequences, or where the nucleic acid does not encode an amino acid sequence, to essentially identical sequences. Because of the degeneracy of the genetic code, a large number of functionally identical nucleic acids encode any given protein. For instance, the codons GCA, GCC, GCG and GCU all encode the amino acid alanine. Thus, at every position where an alanine is specified by a codon, the codon can be altered to any of the corresponding codons described without altering the encoded polypeptide. Such nucleic acid variations are xe2x80x9csilent variations,xe2x80x9d which are one species of conservatively modified variations. Every nucleic acid sequence herein which encodes a polypeptide also describes every possible silent variation of the nucleic acid. One of skill will recognize that each codon in a nucleic acid (except AUG, which is ordinarily the only codon for methionine) can be modified to yield a functionally identical molecule. Accordingly, each silent variation of a nucleic acid which encodes a polypeptide is implicit in each described sequence.
As to amino acid sequences, one of skill will recognize that individual substitutions, deletions or additions to a nucleic acid, peptide, polypeptide, or protein sequence which alters, adds or deletes a single amino acid or a small percentage of amino acids in the encoded sequence is a xe2x80x9cconservatively modified variantxe2x80x9d where the alteration results in the substitution of an amino acid with a chemically similar amino acid. Conservative substitution tables providing finctionally similar amino acids are well known in the art.
The following six groups each contain amino acids that are conservative substitutions for one another:
1) Alanine (A), Serine (S), Threonine (T);
2) Aspartic acid (D), Glutamic acid (E);
3) Asparagine (N), Glutamine (Q);
4) Arginine (R), Lysine (K);
5) Isoleucine (I), Leucine (L), Methionine (M), Valine (V); and
6) Phenylalanine (F), Tyrosine (Y), Tryptophan (W). (see, e.g., Creighton, Proteins (1984)).
An indication that two nucleic acid sequences or polypeptides are substantially identical is that the polypeptide encoded by the first nucleic acid is immunologically cross reactive with the antibodies raised against the polypeptide encoded by the second nucleic acid. Thus, a polypeptide is typically substantially identical to a second polypeptide, for example, where the two peptides differ only by conservative substitutions. Another indication that two nucleic acid sequences are substantially identical is that the two molecules or their complements hybridize to each other under stringent conditions, as described below.
The phrase xe2x80x9cselectively (or specifically) hybridizes toxe2x80x9d refers to the binding, duplexing, or hybridizing of a molecule only to a particular nucleotide sequence under stringent hybridization conditions when that sequence is present in a complex mixture (e.g., total cellular or library DNA or RNA).
The phrase xe2x80x9cstringent hybridization conditionsxe2x80x9d refers to conditions under which a probe will hybridize to its target subsequence, typically in a complex mixture of nucleic acid, but to no other sequences. Stringent conditions are sequence-dependent and will be different in different circumstances. Longer sequences hybridize specifically at higher temperatures. An extensive guide to the hybridization of nucleic acids is found in Tijssen, Techniques in Biochemistry and Molecular Biologyxe2x80x94Hybridization with Nucleic Probes, xe2x80x9cOverview of principles of hybridization and the strategy of nucleic acid assaysxe2x80x9d (1993). Generally, highly stringent conditions are selected to be about 5-10xc2x0 C. lower than the thermal melting point (Tm) for the specific sequence at a defmed ionic strength pH. Low stringency conditions are generally selected to be about 15-30xc2x0 C. below the Tm. The Tm is the temperature (under defined ionic strength, pH, and nucleic concentration) at which 50% of the probes complementary to the target hybridize to the target sequence at equilibrium (as the target sequences are present in excess, at Tm, 50% of the probes are occupied at equilibrium). Stringent conditions will be those in which the salt concentration is less than about 1.0 M sodium ion, typically about 0.01 to 1.0 M sodium ion concentration (or other salts) at pH 7.0 to 8.3 and the temperature is at least about 30xc2x0 C. for short probes (e.g., 10 to 50 nucleotides) and at least about 60xc2x0 C. for long probes (e.g., greater than 50 nucleotides). Stringent conditions may also be achieved with the addition of destabilizing agents such as fonnamide. For selective or specific hybridization, a positive signal is at least two times background, preferably 10 time background hybridization.
Nucleic acids that do not hybridize to each other under stringent conditions are still substantially identical if the polypeptides which they encode are substantially identical. This occurs, for example, when a copy of a nucleic acid is created using the maximum codon degeneracy permitted by the genetic code. In such cased, the nucleic acids typically hybridize under moderately stringent hybridization conditions.
In the present invention, genomic DNA or cDNA comprising ATR nucleic acids of the invention can be identified in standard Southern blots under stringent conditions using the nucleic acid sequences disclosed here. For the purposes of this disclosure, suitable stringent conditions for such hybridizations are those which include a hybridization in a buffer of 40% formamide, 1 M NaCl, 1% SDS at 37xc2x0 C., and at least one wash in 0.2xc3x97SSC at a temperature of at least about 50xc2x0 C., usually about 55xc2x0 C. to about 60xc2x0 C., for 20 minutes, or equivalent conditions. A positive hybridization is at least twice background. Those of ordinary skill will readily recognize that alternative hybridization and wash conditions can be utilized to provide conditions of similar stringency.
A further indication that two polynucleotides are substantially identical is if the reference sequence, amplified by a pair of oligonucleotide primers, can then be used as a probe under stringent hybridization conditions to isolate the test sequence from a cDNA or genomic library, or to identify the test sequence in, e.g., a northern or Southern blot.
This invention provides molecular strategies for controlling seed and fruit development, and in particular, endosperm development.
Reproduction in flowering plants involves two fertilization events in the haploid female gametophyte. One sperm nucleus fertilizes the egg to form the embryo. A second sperm nucleus fertilizes the central cell to form the endosperm, a unique tissue that supports the growth of the embryo. Fertilization also activates maternal tissue differentiation, the ovule integuments form the seed coat and the ovary forms the fruit.
The present invention is based, at least in part, on the discovery of a set of female-gametophytic mutations and the subsequent cloning of the gene, termed ATROPOS (ATR), involved. Two mutant alleles of ATR disclosed here were created using a T-DNA tag, thereby disrupting an exon of the gene. The atr mutations affect endosperm production, allowing for increased endosperm development. Generally, the mutant atr alleles are not transmitted by the female gametophyte. Inheritance of a mutant atr allele by the female gametophyte usually results in embryo abortion and endosperm overproduction, even when the pollen bears the wild-type ATR allele.
In contrast, transmission of atr mutant alleles through the male gametophyte (i.e., pollen) is ecotype-dependent in Arabidopsis. For instance, in some ecotypes (e.g., Columbia), transmission of atr mutant alleles is less than 50%. However, in Landsberg erecta, transmission is almost normal.
The isolated sequences prepared as described herein, can be used in a number of techniques, for example, to suppress or enhance endogenous ATR gene expression. Modulation of ATR gene expression or ATR activity in plants is particularly useful, for example, in producing embryo-less or embryo-reduced seed, seed with increased endosperm, or as part of a system to generate seed.
Isolation of ATR Nucleic Acids
Generally, the nomenclature and the laboratory procedures in recombinant DNA technology described below are those well known and commonly employed in the art. Standard techniques are used for cloning, DNA and RNA isolation, amplification and purification. Generally enzymatic reactions involving DNA ligase, DNA polymerase, restriction endonucleases and the like are performed according to the manufacturer""s specifications. These techniques and various other techniques are generally performed according to Sambrook et al., Molecular Cloningxe2x80x94A Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y., (1989).
The isolation of ATR nucleic acids may be accomplished by a number of techniques. For instance, oligonucleotide probes based on the sequences disclosed here can be used to identify the desired gene in a cDNA or genomic DNA library. To construct genomic libraries, large segments of genomic DNA are generated by random fragmentation, e.g. using restriction endonucleases, and are ligated with vector DNA to form concatemers that can be packaged into the appropriate vector. To prepare a cDNA library, mRNA is isolated from the desired organ, such as ovules, and a cDNA library which contains the ATR gene transcript is prepared from the mRNA. Alternatively, cDNA may be prepared from mRNA extracted from other tissues in which ATR genes or homologs are expressed.
The cDNA or genomic library can then be screened using a probe based upon the sequence of a cloned ATR gene disclosed here. Probes may be used to hybridize with genomic DNA or cDNA sequences to isolate homologous genes in the same or different plant species. Alternatively, antibodies raised against an ATR polypeptide can be used to screen an mRNA expression library.
Alternatively, the nucleic acids of interest can be amplified from nucleic acid samples using amplification techniques. For instance, polymerase chain reaction (PCR) technology can be used to amplify the sequences of the ATR genes directly from genomic DNA, from cDNA, from genomic libraries or cDNA libraries. PCR and other in vitro amplification methods may also be useful, for example, to clone nucleic acid sequences that code for proteins to be expressed, to make nucleic acids to use as probes for detecting the presence of the desired mRNA in samples, for nucleic acid sequencing, or for other purposes. For a general overview of PCR see PCR Protocols: A Guide to Methods and Applications. (Innis, M, Gelfand, D., Sninsky, J. and White, T., eds.), Academic Press, San Diego (1990).
Appropriate primers and probes for identifying A TR sequences from plant tissues are generated from comparisons of the sequences provided here with other related genes. For instance, ATR can be compared to the other endonuclease III genes, such as Genbank Accession No. AE002073. Using these techniques, one of skill can identify conserved regions in the nucleic acids disclosed here to prepare the appropriate primer and probe sequences. Primers that specifically hybridize to conserved regions in ATR genes can be used to amplify sequences from widely divergent plant species. Appropriate primers for amplification of the genomic region or cDNA of LEC2 include the following primers:
Xba-SKEN-7; CCTCTAGAGGAATTGTCGGCAAAATCGAG (SEQ ID NO:7)
SKB-8; GGAGAGACGGTTATTGTCAACC (SEQ ID NO:8)
SKB-7; AAAAGTCTACAAGGGAGAGAGAGT (SEQ ID NO:9)
SKB-5; GTAGATGTACATACGTACC (SEQ ID NO10)
SKEN-8; GCATCCTCCAACAAGTAACAATCCACTC (SEQ ID NO:11)
SKB-6; CACTGAGATTAATTCTTCAGACTCG (SEQ ID NO:12)
SKEN-3.5; CTCAGGCGAGTCAATGCCGGAGAACAC (SEQ ID NO:13)
SKEN-3; CGAGGGCTGATCCGGGGGATAGATATTTT (SEQ ID NO:14)
SKEN-2; CCCCCGGATCAGCCCTCGAATTC (SEQ ID NO:15)
SKEN-1;CCCCTGTCTACAAATTCACCACCTGG (SEQ ID NO:16)
SKEL-4; CTGACCCAACTGCTTCTCTTC (SEQ ID NO:17)
skes1.5; TCACCTGTTCTGAACAGACTGG (SEQ ID NO:18)
SKES-1.4; CAGCAGACGAGTCCATAATGCTCTGC (SEQ ID NO:19)
SKES-2.4; GGTTTGCCTTCCACGACCACC (SEQ ID NO:20)
SKES-1; GGAAGCCACGCAAAGCTGCAACTCAGG (SEQ ID NO:21)
SKES-2.45; GAGTTGCAGCTTTGCGTGGCTTCC (SEQ ID NO:22)
SKES2.5; TTCAGACTCAGAGTCACCTTGC (SEQ ID NO:23)
SKES-2; ACCAGCAGCCTTGCTTGGCC (SEQ ID NO:24)
SKES-3; CATGCCAGAGAAGCAGGGCTCC (SEQ ID NO:25)
SKES3.5; CGATGATACTGTCTCTTCGAGC (SEQ ID NO:26)
SKES-6; CCTCCGCCTGCTCATGCCTCAG (SEQ ID NO:27)
SKEN-4; GTCCATCAGGAGAACTTCTGTGTCAGGAT (SEQ ID NO:28)
SKES-4; GGGAACAAGTGCACCATCTCC (SEQ ID NO:29)
SKEN-6; GCTCTCATAGGGAACAAGTGCACCATCTC (SEQ ID NO:30)
SKES-5; CGCTCGCATGCACCTGGTAC (SEQ ID NO:31)
SKB-1;GGAGGGAATCGAGCAGCTAGAG (SEQ ID NO:32)
SKB-2; GAGCAGCTAAGGGACTGTTCAAACTC (SEQ ID NO:33)
SKB-3; CCAGGAATGGGATTGTCCGG (SEQ ID NO:34)
3xe2x80x2RACE-2; CTTGGACGGCGCTTGAGGAACC (SEQ ID NO:35)
3xe2x80x2RACE-1; GCCTACAAGCCAGTGGGATAG (SEQ ID NO:36)
cDNA-1; GCCAAGGACTATCTCTTGAGC (SEQ ID NO:37)
SKB-4;GGATGGACTCGAGCACTGGG (SEQ ID NO:38)
SKE2.2-4; AGAGGAGAGTGCAGACACTTTG (SEQ ID NO:39)
cDNA-3; GAGGACCCTGACGAGATCCCAAC (SEQ ID NO:40)
cDNA-9; CCATGTGTTCCCGTAGAGTCATTCC (SEQ ID NO:41)
2.2+SKE-1; ATGGAGCTCCAAGAAGGTGACATG (SEQ ID NO:42)
cDNA-5; CAGAAGTGTGGAGGGAAAGCGTCTGGC (SEQ ID NO:43)
cDNA-4; CCCTCAGACTGTTACACTCAGAAC (SEQ ID NO:44)
cDNA-2; CCCGTTGAGCGGAAAACTTCCTCTCATGGC (SEQ ID NO:45)
cDNA-7; GGAAAGGATTCGTATGTGTCCGTGG (SEQ ID NO:46)
SKEN-5; GCAATGCGTTTGCTTTCTTCCAGTCATCT (SEQ ID NO:47)
cDNA-6; GAGGAGAGCAGAGAAGCAATGCGTTTGC (SEQ ID NO:48)
cDNA-8; GTTAGAGAGAAAATAAATAACCC (SEQ ID NO:49)
2.2+SKE-3; CCGTAAACAACACCGGATACAC (SEQ ID NO:50)
The amplification conditions are typically as follows. Reaction components: 10 mM Tris-HCl, pH 8.3, 50 mM potassium chloride, 1.5 mM magnesium chloride, 0.001% gelatin, 200 xcexcM dATP, 200 xcexcM dCTP, 200 xcexcM dGTP, 200 xcexcM dTTP, 0.4 xcexcM primers, and 100 units per ml Taq polymerase. Program: 96 C for 3 min., 30 cycles of 96 C for 45 sec., 50 C for 60 sec., 72 for 60 sec, followed by 72 C for 5 min.
Standard nucleic acid hybridization techniques using the conditions disclosed above can then be used to identify full-length CDNA or genomic clones.
Control of ATR activity or Gene Expression
Since ATR genes are involved in controlling seed, in particular endosperm, development, inhibition of endogenous ATR activity or gene expression is useful in a number of contexts. For instance, reduction of ATR activity can be used for production of seed with enhanced endosperm. By reducing and/or eliminating ATR activity, plants with seed containing increased endosperm can be produced.
Alternatively, substantial inhibition of ATR activity can be used for production of fruit with small and/or degraded seed (referred to here as xe2x80x9cseedless fruitxe2x80x9d) after fertilization. In many plants, particularly dicots, the endosperm is not persistent and eventually is degraded. Thus, in plants of the invention in which ATR activity is inhibited, embryo-less seed do not persist and seedless fruit are produced. For production of dicots with enhanced endosperm, the most beneficial effect may be to reduce, but not eliminate ATR activity. On the other hand, in monocots, which have persistent endosperm, it is advantageous to eliminate ATR activity.
Alternatively, plants of the invention can be used to prevent pre-harvest sprouting in seeds, especially those derived from cereals. In these plants, the endosperm persists and is the major component of the mature seed. Premature growth of embryos in stored grain causes release of degradative enzymes which digest starch and other components of the endosperm. Plants of the present invention are useful in addressing this problem because the seeds lack an embryo and thus will not germinate.
One of skill will recognize that a number of methods can be used to modulate ATR activity or gene expression. ATR activity can be modulated in the plant cell at the gene, transcriptional, posttranscriptional, translational, or posttranslational, levels. Techniques for modulating ATR activity at each of these levels are generally well known to one of skill and are discussed briefly below.
Methods for introducing genetic mutations into plant genes are well known. For instance, seeds or other plant material can be treated with a mutagenic chemical substance, according to standard techniques. Such chemical substances include, but are not limited to, the following: diethyl sulfate, ethylene imine, ethyl methanesulfonate and N-nitroso-N-ethylurea. Alternatively, ionizing radiation from sources such as, for example, X-rays or gamma rays can be used.
Alternatively, homologous recombination can be used to induce targeted gene disruptions by specifically deleting or altering the ATR gene in vivo (see, generally, Grewal and Klar, Genetics 146: 1221-1238 (1997) and Xu et al., Genes Dev. 10:2411-2422 (1996)). Homologous recombination has been demonstrated in plants (Puchta et al., Experientia 50:277-284 (1994), Swoboda et al., EMBO J. 13:484-489 (1994); Offringa et al., Proc. Natl. Acad. Sci. USA 90: 7346-7350 (1993); and Kempin et al. Nature 389:802-803 (1997)).
In applying homologous recombination technology to the genes of the invention, mutations in selected portions of an ATR gene sequences (including 5xe2x80x2 upstream, 3xe2x80x2 downstream, and intragenic regions) such as those disclosed here are made in vitro and then introduced into the desired plant using standard techniques. Since the efficiency of homologous recombination is known to be dependent on the vectors used, use of dicistronic gene targeting vectors as described by Mountford et al. Proc. Natl. Acad. Sci. USA 91:4303-4307 (1994); and Vaulont et al. Transgenic Res. 4:247-255 (1995) are conveniently used to increase the efficiency of selecting for altered ATR gene expression in transgenic plants. The mutated gene will interact with the target wild-type gene in such a way that homologous recombination and targeted replacement of the wild-type gene will occur in transgenic plant cells, resulting in suppression of ATR activity.
Alternatively, oligonucleotides composed of a contiguous stretch of RNA and DNA residues in a duplex conformation with double hairpin caps on the ends can be used. The RNA/DNA sequence is designed to align with the sequence of the target ATR gene and to contain the desired nucleotide change. Introduction of the chimeric oligonucleotide on an extrachromosomal T-DNA plasmid results in efficient and specific ATR gene conversion directed by chimeric molecules in a small number of transformed plant cells. This method is described in Cole-Strauss et al. Science 273:1386-1389 (1996) and Yoon et al. Proc. Natl. Acad. Sci. USA 93:2071-2076 (1996).
Gene expression can be inactivated using recombinant DNA techniques by transforming plant cells with constructs comprising transposons or T-DNA sequences. ATR mutants prepared by these methods are identified according to standard techniques. For instance, mutants can be detected by PCR or by detecting the presence or absence of ATR mRNA, e.g., by Northern blots. Mutants can also be selected by assaying for development of endosperm in the absence of fertilization.
The isolated nucleic acid sequences prepared as described herein, can also be used in a number of techniques to control endogenous ATR gene expression at various levels. Subsequences from the sequences disclosed here can be used to control, transcription, RNA accumulation, translation, and the like.
A number of methods can be used to inhibit gene expression in plants. For instance, antisense technology can be conveniently used. To accomplish this, a nucleic acid segment from the desired gene is cloned and operably linked to a promoter such that the antisense strand of RNA will be transcribed. The construct is then transformed into plants and the antisense strand of RNA is produced. In plant cells, it has been suggested that antisense suppression can act at all levels of gene regulation including suppression of RNA translation (see, Bourque Plant Sci. (Limerick) 105:125-149 (1995); Pantopoulos In Progress in Nucleic Acid Research and Molecular Biology, Vol. 48. Cohn, W. E. and K. Moldave (Ed.). Academic Press, Inc.: San Diego, Calif., USA; London, England, UK. p. 181-238; Heiser et al. Plant Sci. (Shannon) 127:61-69 (1997)) and by preventing the accumulation of mRNA which encodes the protein of interest, (see, Baulcombe Plant Mol. Bio. 32:79-88 (1996); Prins and Goldbach Arch. Virol. 141:2259-2276 (1996); Metzlaffet al. Cell 88:845-854 (1997), Sheehy et al., Proc. Nat. Acad. Sci. USA, 85:8805-8809 (1988), and Hiatt et al., U.S. Pat. No. 4,801,340).
The nucleic acid segment to be introduced generally will be substantially identical to at least a portion of the endogenous ATR gene or genes to be repressed. The sequence, however, need not be perfectly identical to inhibit expression. The vectors of the present invention can be designed such that the inhibitory effect applies to other genes within a family of genes exhibiting homology or substantial homology to the target gene.
For antisense suppression, the introduced sequence also need not be full length relative to either the primary transcription product or fully processed mRNA. Generally, higher homology can be used to compensate for the use of a shorter sequence. Furthermore, the introduced sequence need not have the same intron or exon pattern, and homology of non-coding segments may be equally effective. Normally, a sequence of between about 30 or 40 nucleotides and about full length nucleotides should be used, though a sequence of at least about 100 nucleotides is preferred, a sequence of at least about 200 nucleotides is more preferred, and a sequence of about 500 to about 7000 nucleotides is especially preferred.
A number of gene regions can be targeted to suppress ATR gene expression. The targets can include, for instance, the coding regions, introns, sequences from exon/intron junctions, 5xe2x80x2 or 3xe2x80x2 untranslated regions, and the like. In some embodiments, the constructs can be designed to eliminate the ability of regulatory proteins to bind to ATR gene sequences that are required for its cell- and/or tissue-specific expression. Such transcriptional regulatory sequences can be located either 5xe2x80x2-, 3xe2x80x2-, or within the coding region of the gene and can be either promote (positive regulatory element) or repress (negative regulatory element) gene transcription. These sequences can be identified using standard deletion analysis, well known to those of skill in the art. Once the sequences are identified, an antisense construct targeting these sequences is introduced into plants to control gene transcription in particular tissue, for instance, in developing ovules and/or seed. In one embodiment, transgenic plants are selected for ATR activity that is reduced but not eliminated.
Oligonucleotide-based triple-helix formation can be used to disrupt ATR gene expression. Triplex DNA can inhibit DNA transcription and replication, generate site-specific mutations, cleave DNA, and induce homologous recombination (see, e.g., Havre and Glazer J. Virology 67:7324-7331 (1993); Scanlon et al. FASEB J. 9:1288-1296 (1995); Giovannangeli et al. Biochemistry 35:10539-10548 (1996); Chan and Glazer J. Mol. Medicine (Berlin) 75:267-282 (1997)). Triple helix DNAs can be used to target the same sequences identified for antisense regulation.
Catalytic RNA molecules or ribozymes can also be used to inhibit expression ofATR genes. It is possible to design ribozymes that specifically pair with virtually any target RNA and cleave the phosphodiester backbone at a specific location, thereby functionally inactivating the target RNA. In carrying out this cleavage, the ribozyme is not itself altered, and is thus capable of recycling and cleaving other molecules, making it a true enzyme. The inclusion of ribozyme sequences within antisense RNAs confers RNA-cleaving activity upon them, thereby increasing the activity of the constructs. Thus, ribozymes can be used to target the same sequences identified for antisense regulation.
A number of classes of ribozymes have been identified. One class of ribozymes is derived from a number of small circular RNAs which are capable of self-cleavage and replication in plants. The RNAs replicate either alone (viroid RNAs) or with a helper virus (satellite RNAs). Examples include RNAs from avocado sunblotch viroid and the satellite RNAs from tobacco ringspot virus, lucerne transient streak virus, velvet tobacco mottle virus, solanum nodiflorum mottle virus and subterranean clover mottle virus. The design and use of target RNA-specific ribozymes is described in Zhao and Pick Nature 365:448-451 (1993); Eastham and Ahlering J. Urology 156:1186-1188 (1996); Sokol and Murray Transgenic Res. 5:363-371 (1996); Sun et al. Mol. Biotechnology 7:241-251 (1997); and Haseloff et al. Nature, 334:585-591 (1988).
Another method of suppression is sense cosuppression. Introduction of nucleic acid configured in the sense orientation has been recently shown to be an effective means by which to block the transcription of target genes. For an example of the use of this method to modulate expression of endogenous genes (see, Assaad et al. Plant Mol. Bio. 22:1067-1085 (1993); Flavell Proc. Natl. Acad. Sci. USA 91:3490-3496 (1994); Stam et al. Annals Bot. 79:3-12 (1997); Napoli et al., The Plant Cell 2:279-289 (1990); and U.S. Pat. Nos. 5,034,323, 5,231,020, and 5,283,184).
The suppressive effect may occur where the introduced sequence contains no coding sequence per se, but only intron or untranslated sequences homologous to sequences present in the primary transcript of the endogenous sequence. The introduced sequence generally will be substantially identical to the endogenous sequence intended to be repressed. This minimal identity will typically be greater than about 65%, but a higher identity might exert a more effective repression of expression of the endogenous sequences. Substantially greater identity of more than about 80% is preferred, though about 95% to absolute identity would be most preferred. As with antisense regulation, the effect should apply to any other proteins within a similar family of genes exhibiting homology or substantial homology.
For sense suppression, the introduced sequence, needing less than absolute identity, also need not be full length, relative to either the primary transcription product or fully processed MRNA. This may be preferred to avoid concurrent production of some plants that are overexpressers. A higher identity in a shorter than full length sequence compensates for a longer, less identical sequence. Furthermore, the introduced sequence need not have the same intron or exon pattern, and identity of non-coding segments will be equally effective. Normally, a sequence of the size ranges noted above for antisense regulation is used. In addition, the same gene regions noted for antisense regulation can be targeted using cosuppression technologies.
In a preferred embodiment, expression of a nucleic acid of interest can be suppressed by the simultaneous expression of both sense and antisense constructs (Waterhouse et al., Proc. Natl. Acad. Sci. USA 95:13959-13964 (1998). See also Tabara et al. Science 282:430-431 (1998).
Alternatively, ATR activity may be modulated by eliminating the proteins that are required for ATR cell-specific gene expression. Thus, expression of regulatory proteins and/or the sequences that control ATR gene expression can be modulated using the methods described here.
Another method is use of engineered tRNA suppression of ATR MRNA translation. This method involves the use of suppressor tRNAs to transactivate target genes containing premature stop codons (see, Betzner et al. Plant J. 11:587-595 (1997); and Choisne et al. Plant J. 11:597-604 (1997). A plant line containing a constitutively expressed ATR gene that contains an amber stop codon is first created. Multiple lines of plants, each containing tRNA suppressor gene constructs under the direction of cell-type specific promoters are also generated. The tRNA gene construct is then crossed into the ATR line to activate ATR activity in a targeted manner. These tRNA suppressor lines could also be used to target the expression of any type of gene to the same cell or tissue types.
ATR proteins may form homogeneous or heterologous complexes in vivo. Thus, production of dominant-negative forms of ATR polypeptides that are defective in their abilities to bind to other proteins in the complex is a convenient means to inhibit endogenous ATR activity. This approach involves transformation of plants with constructs encoding mutant ATR polypeptides that form defective complexes and thereby prevent the complex from forming properly. The mutant polypeptide may vary from the naturally occurring sequence at the primary structure level by amino acid substitutions, additions, deletions, and the like. These modifications can be used in a number of combinations to produce the final modified protein chain. Use of dominant negative mutants to inactivate target genes is described in Mizukami et al. Plant Cell 8:831-845 (1996).
Another strategy to affect the ability of an ATR protein to interact with itself or with other proteins involves the use of antibodies specific to ATR. In this method cell-specific expression of ATR-specific Abs is used inactivate fimctional domains through antibody:antigen recognition (see, Hupp et al. Cell 83:237-245 (1995)).
After plants with reduced ATR activity are identified, a recombinant construct capable of expressing low levels of ATR in embryos can be introduced using the methods discussed below. In this fashion, the level of ATR activity can be regulated to produce preferred plant phenotypes. For example, a relatively weak promoter such as the ubiquitin promoter (see, e.g., Garbarino et al. Plant Physiol. 109(4):1371-8 (1995); Christensen et al Transgenic Res. 5(3):213-8 (1996); and Holtorf et al. Plant. Mol. Biol. 29(4):637-46 (1995)) is useful to produce plants with reduced levels of ATR activity or expression. Such plants are useful for producing, for instance, plants that produce seed with enhanced endosperm.
Use of Nucleic Acids of the Invention to Enhance ATR Gene Expression
Isolated sequences prepared as described herein can also be used to introduce expression of a particular ATR nucleic acid to enhance or increase endogenous gene expression. For instance, without being bound to any theory, in light of ATR""s relation to Exonuclease III, applicants believe that ATR binds DNA or chromatin and acts to modulate transcription. Enhanced expression can therefore be used to control plant morphology by controlling expression of genes under ATR""s control in desired tissues or cells. Enhanced expression can also be used, for instance, to increase vegetative growth by preventing the plant from setting seed. Where overexpression of a gene is desired, the desired gene from a different species may be used to decrease potential sense suppression effects.
One of skill will recognize that the polypeptides encoded by the genes of the invention, like other proteins, have different domains that perform different functions. Thus, the gene sequences need not be full length, so long as the desired finctional domain of the protein is expressed.
Modified protein chains can also be readily designed utilizing various recombinant DNA techniques well known to those skilled in the art and described in detail, below. For example, the chains can vary from the naturally occurring sequence at the primary structure level by amino acid substitutions, additions, deletions, and the like. These modifications can be used in a number of combinations to produce the final modified protein chain.
Preparation of Recombinant Vectors
To use isolated sequences in the above techniques, recombinant DNA vectors suitable for transformation of plant cells are prepared. Techniques for transforming a wide variety of flowering plant species are well known and described in the technical and scientific literature. See, for example, Weising et al. Ann. Rev. Genet. 22:421-477 (1988). A DNA sequence coding for the desired polypeptide, for example a cDNA sequence encoding a full length protein, will preferably be combined with transcriptional and translational initiation regulatory sequences which will direct the transcription of the sequence from the gene in the intended tissues of the transformed plant.
For example, for overexpression, a plant promoter fragment may be employed which will direct expression of the gene in all tissues of a regenerated plant. Such promoters are referred to herein as xe2x80x9cconstitutivexe2x80x9d promoters and are active under most environmental conditions and states of development or cell differentiation. Examples of constitutive promoters include the cauliflower mosaic virus (CaMV) 35S transcription initiation region, the 1xe2x80x2- or 2xe2x80x2-promoter derived from T-DNA of Agrobacterium tumafaciens, and other transcription initiation regions from various plant genes known to those of skill. Such genes include for example, ACT11 from Arabidopsis (Huang et al. Plant Mol. Biol. 33:125-139 (1996)), Cat3 from Arabidopsis (GenBank No. U43147, Zhong et al., Mol. Gen. Genet. 251:196-203 (1996)), the gene encoding stearoyl-acyl carrier protein desaturase from Brassica napus (Genbank No. X74782, Solocombe et al. Plant Physiol. 104:1167-1176 (1994)), GPc1 from maize (GenBank No. X15596, Martinez et al. J. Mol. Biol 208:551-565 (1989)), and Gpc2 from maize (GenBank No. U45855, Manjunath et al., Plant Mol. Biol. 33:97-112 (1997)).
Alternatively, the plant promoter may direct expression of the ATR nucleic acid in a specific tissue or may be otherwise under more precise environmental or developmental control. Examples of environmental conditions that may effect transcription by inducible promoters include anaerobic conditions, elevated temperature, or the presence of light. Such promoters are referred to here as xe2x80x9cinduciblexe2x80x9d or xe2x80x9ctissue-specificxe2x80x9d promoters. One of skill will recognize that a tissue-specific promoter may drive expression of operably linked sequences in tissues other than the target tissue. Thus, as used herein a tissue-specific promoter is one that drives expression preferentially in the target tissue, but may also lead to some expression in other tissues as well.
Examples of promoters under developmental control include promoters that initiate transcription only (or primarily only) in certain tissues, such as fruit, seeds, or flowers. Promoters that direct expression of nucleic acids in ovules, flowers or seeds are particularly useful in the present invention. As used herein a seed-specific promoter is one which directs expression in seed tissues, such promoters may be, for example, ovule-specific (which includes promoters which direct expression in maternal tissues or the female gametophyte, such as egg cells or the central cell), embryo-specific, endosperm-specific, integument-specific, seed coat-specific, or some combination thereof. Examples include a promoter from the ovule-specific BEL1 gene described in Reiser et al. Cell 83:735-742 (1995) (GenBank No. U39944). Other suitable seed specific promoters are derived from the following genes: MAC1 from maize (Sheridan et al. Genetics 142:1009-1020 (1996), Cat3 from maize (GenBank No. L05934, Abler et al. Plant Mol. Biol. 22:10131-1038 (1993), the gene encoding oleosin 18 kD from maize (GenBank No. J05212, Lee et al. Plant Mol. Biol. 26:1981-1987 (1994)), vivparous-1 from Arabidopsis (Genbank No. U93215), the gene encoding oleosin from Arabidopsis (Genbank No. Z17657), Atmycl from Arabidopsis (Urao et al. Plant Mol. Biol. 32:571-576 (1996), the 2s seed storage protein gene family from Arabidopsis (Conceicao et al. Plant 5:493-505 (1994)) the gene encoding oleosin 20 kD from Brassica napus (GenBank No. M63985), napA from Brassica napus (GenBank No. J02798, Josefsson et al. JBL 26:12196-1301 (1987), the napin gene family from Brassica napus (Sjodahl et al. Planta 197:264-271 (1995), the gene encoding the 2S storage protein from Brassica napus (Dasgupta et al. Gene 133:301-302 (1993)), the genes encoding oleosin A (Genbank No. U09118) and oleosin B (Genbank No. U09119) from soybean and the gene encoding low molecular weight sulphur rich protein from soybean (Choi et al. Mol Gen, Genet. 246:266-268 (1995)).
In addition, the promoter sequences from the ATR genes disclosed here can be used to drive expression of the ATR polynucleotides of the invention or heterologous sequences. The sequences of the promoters are identified below.
If proper polypeptide expression is desired, a polyadenylation region at the 3xe2x80x2-end of the coding region should be included. The polyadenylation region can be derived from the natural gene, from a variety of other plant genes, or from T-DNA.
The vector comprising the sequences (e.g., promoters or coding regions) from genes of the invention will typically comprise a marker gene which confers a selectable phenotype on plant cells. For example, the marker may encode biocide resistance, particularly antibiotic resistance, such as resistance to kanamycin, G418, bleomycin, hygromycin, or herbicide resistance, such as resistance to chlorosulfuron or Basta.
Production of Transgenic Plants
DNA constructs of the invention may be introduced into the genome of the desired plant host by a variety of conventional techniques. For example, the DNA construct may be introduced directly into the genomic DNA of the plant cell using techniques such as electroporation and microinjection of plant cell protoplasts, or the DNA constructs can be introduced directly to plant tissue using ballistic methods, such as DNA particle bombardment.
Microinjection techniques are known in the art and well described in the scientific and patent literature. The introduction of DNA constructs using polyethylene glycol precipitation is described in Paszkowski et al. Embo J. 3:2717-2722 (1984). Electroporation techniques are described in Fromm et al. Proc. Natl. Acad. Sci. USA 82:5824 (1985). Ballistic transformation techniques are described in Klein et al. Nature 327:70-73 (1987).
Alternatively, the DNA constructs may be combined with suitable T-DNA flanking regions and introduced into a conventional Agrobacterium tumefaciens host vector. The virulence functions of the Agrobacterium tumefaciens host will direct the insertion of the construct and adjacent marker into the plant cell DNA when the cell is infected by the bacteria. Agrobacterium tumefaciens-mediated transformation techniques, including disarming and use of binary vectors, are well described in the scientific literature. See, for example Horsch et al. Science 233:496-498 (1984), and Fraley et al. Proc. Natl. Acad. Sci. USA 80:4803 (1983).
Transformed plant cells which are derived by any of the above transformation techniques can be cultured to regenerate a whole plant which possesses the transformed genotype and thus the desired phenotype such as increased seed mass. Such regeneration techniques rely on manipulation of certain phytohormones in a tissue culture growth medium, typically relying on a biocide and/or herbicide marker which has been introduced together with the desired nucleotide sequences. Plant regeneration from cultured protoplasts is described in Evans et al., Protoplasts Isolation and Culture, Handbook of Plant Cell Culture, pp. 124-176, MacMillilan Publishing Company, New York, 1983; and Binding, Regeneration of Plants, Plant Protoplasts, pp. 21-73, CRC Press, Boca Raton, 1985. Regeneration can also be obtained from plant callus, explants, organs, or parts thereof. Such regeneration techniques are described generally in Klee et al. Ann. Rev. ofPlant Phys. 38:467-486 (1987).
The nucleic acids of the invention can be used to confer desired traits on essentially any plant. Thus, the invention has use over a broad range of plants, including species from the genera Anacardium, Arachis, Asparagus, Atropa, Avena, Brassica, Citrus, Citrullus, Capsicum, Carthamus, Cocos, Coffea, Cucumis, Cucurbita, Daucus, Elaeis, Fragaria, Glycine, Gossypium, Helianthus, Heterocallis, Hordeum, Hyoscyamus, Lactuca, Linum, Lolium, Lupinus, Lycopersicon, Malus, Manihot, Majorana, Medicago, Nicotiana, Olea, Oryza, Panieum, Pannesetum, Persea, Phaseolus, Pistachia, Pisum, Pyrus, Prunus, Raphanus, Ricinus, Secale, Senecio, Sinapis, Solanum, Sorghum, Theobromus, Trigonella, Triticum, Vicia, Vitis, Vigna, and Zea.
One of skill will recognize that after the expression cassette is stably incorporated in transgenic plants and confirmed to be operable, it can be introduced into other plants by sexual crossing. Any of a number of standard breeding techniques can be used, depending upon the species to be crossed.
Seed obtained from plants of the present invention can be analyzed according to well known procedures to identify plants with the desired trait. If antisense or other techniques are used to control ATR gene expression, Northern blot analysis can be used to screen for desired plants. In addition, the presence of fertilization independent reproductive development can be detected. Plants can be screened, for instance, for the ability to form embryo-less seed, form seed that abort after fertilization, or set fruit in the absence of fertilization. These procedures will depend, part on the particular plant species being used, but will be carried out according to methods well known to those of skill.